1. Field of the Invention
The invention relates to induction heating systems, and more particularly, to apparatus and methods for delivering optimum power to a workpiece over a wide range operating conditions.
2. Description of Related Art
Induction heating systems heat an electrically conductive workpiece by magnetically inducing eddy currents therein. Electrical resistance in the eddy current paths in the workpiece cause I.sup.2 R losses, which in turn heat the workpiece.
One type of induction heating system includes a power supply inverter, which has an AC voltage output having a desired frequency of operation. The output of the inverter is usually connected through a step-down transformer to a pair of power supply output terminals, across which is connected the series combination of a series inductor and a resonant tank circuit. The tank circuit includes a work coil in parallel combination with a resonance capacitor. The work coil, in operation, is placed in proximity with the workpiece, and creates the oscillating magnetic field which induces the eddy currents in the workpiece.
Depending on the application, a wide variety of different operating conditions may be desired. For example, different applications may require different frequencies of operation. Frequencies commonly used for induction heating range anywhere from approximately 10 kHz to approximately 400 kHz. Different applications can also require different voltages across the work coil. Additionally, depending on the configuration and composition of the workpiece, the power factor of the energy delivered to the work coil could also vary widely.
Most induction heating systems are designed for a particular application. For example, a system designed to heat automobile bodies for the purpose of drying paint that has been applied to the surface, need only be designed to operate at one particular frequency, voltage and power factor. It is desirable, however, to provide a general-purpose induction heating system which can be used in a wide variety of applications, under a wide variety of different circumstances. For example, it would be desirable to permit a user to select the operational frequency over the full range of typical frequencies, 10 kHz-400 kHz. Adjustability within this large range of frequencies, spanning a range of 40:1, is extremely difficult to support. Even a range of 50 kHz-400 kHz (8:1) is very difficult to support. It is desirable to provide a system which supports a large range of operating conditions.
In addition, systems which do support a range of operating conditions typically require an operator to tune the system prior to operation. Tuning procedures for such systems are typically complicated and require a technical understanding of the principles under which the induction heating system operates. Accordingly, skilled or trained operators are usually required to operate induction heating systems intended to support a variety of operating conditions. It is therefore desirable to provide an induction heating system and method which simplifies the tuning process.